Intervertebral implants including a wedge which is inserted between the spinous processes which extend the posterior part of two consecutive vertebrae and which limits movement towards each other of the two vertebrae are well known in the art.
Degenerative pathologies of the intervertebral disc cause the vertebrae to move towards each other, possibly in the extent of coming into contact, which can pinch the roots of nerves routed laterally between the vertebrae. To remedy this, a wedge is fixed between the spinous processes of the two consecutive vertebrae that can come into contact on movement of the spine, using appropriate fixing means. The wedge therefore blocks the movement of the vertebrae towards each other, in particular when the spine it extended.
The wedges are made from a rigid biocompatible alloy, usually one based on titanium, and cannot be deformed by the forces that are applied to them, unlike the normal intervertebral disc, which is elastically deformable within certain limits. Thus although the wedges prevent contact between two vertebrae, they constitute an incomplete replacement for the intervertebral disc, which allows relative movement of the vertebrae.
To obtain wedges that are elastically deformable, wedges have been envisaged having two opposite rigid material members forming grooves in which the spinous processes of the two vertebrae engage, the two members forming the grooves being connected together by two elastically deformable leaf spring portions. Thus the wedge as a whole is elastically deformable, which gives it properties adapted to reproduce the normal physiological conditions of relative movement of the vertebrae.
However, the above type of wedge is relatively complicated to manufacture and the materials of the leaf springs are usually not biocompatible. What is more, the springs must be relatively large to achieve the necessary elasticity, and then take up a great deal of room.
Accordingly, wedges have been envisaged that are easier to produce, being made in one piece from a rigid material obtained by polymerization and having a modulus of elasticity much lower than the modulus of elasticity of titanium. However, the modulus of elasticity of the material must be sufficient for the grooves of said wedges to be able to immobilize the spinous processes correctly, and for the reason, although they are more deformable than titanium alloy wedges, they are not sufficiently deformable to fulfill their function.